When males are bred to have longer penises, females respond by evolving bigger brains. That is the case for eastern mosquitofish at least, which appear to develop advanced cognitive skills in order to avoid unwanted attention from these well-endowed males.

A team of researchers from Sweden and Australia were looking to understand sexual conflict and its effect on brain evolution. Throughout the animal kingdom, evolutionary conflicts create differing selective pressures for males and females.

Among many species, males force females to mate – known as coercive mating. Female selection, as a result, involves finding ways to avoid the reproductive interests of the males – females can suffer physical damage through these encounters. To do this, they develop cognitive abilities to detect males sooner, so they can escape the encounter.

In a study published in the Proceedings of the Royal Society B: Biological Sciences, researchers were looking to find out how both male and female eastern mosquitofish (Gambusia holbrooki) would respond when the male was bred to have longer gonopodium (a modified anal fin fish use to deposit sperm). Previous research had shown G. holbrooki with longer gonopodium had greater reproductive success.

Gambusia holbrookietrusko25/Flickr

Through artificial selection, they developed three groups of male G. hobrooki – one with longer gonopodium, one control group and a third with shorter genitals. They then placed them with females and let them breed.

Findings showed – unexpectedly – that in the group of longer gonopodium, female brain sizes in later generations had increased. They were 6.5% larger than those in the shorter gonopodium group, and 4.6% bigger than the control group.

"At first, this sex difference seems surprising," the team wrote. "Similar to the cognitive coevolution reported between predator and prey species, we expected the sexes to be in a cognitive arms race over male coercion attempts. As larger brains confer better cognitive abilities, this should have resulted in the evolution of larger male and female brains, but this was not corroborated by our results."

Researchers believe the female fish evolved better cognitive abilities so they could better evade the male mating attempts: "In G. holbrooki, males usually sneak up to females, attempting up to a thousand forced copulations per day. In such a scenario gonopodium size should predict mating success. A larger brain might allow females to better predict their environment, detect males earlier and/or have faster reactions to more successfully evade males. Females with better cognitive skills may even exert mate choice by escaping the mating attempts of certain males."

Concluding, they say sexual conflict is likely an important factor in brain evolution where coercive mating takes place, but say further research will be needed to look at this in relation to different reproductive strategies.